Dr. G. Dickie on the Physiology of Fecundation in-Plants. 7 
adhesion to the other coats, I found the tubes to enter the mouth 
of the secundine, but after repeated observations was unable to 
detect any such connexion as that described by Meyen to occur 
between the end of the pollen-tube and the embryo-sac in some 
species of Mesembryanthemum, &c.; instead of which, the tube 
and the apex of the nucleus (as at present I suppose it to be) 
seemed perfectly continuous, the one being apparently a gradual 
prolongation of the other. This therefore induced me to ex- 
amine the tubes in their whole course, and there were seen what 
I supposed to be the same tubes, terminating upwards in blind 
extremities. In one capsule the pollen grains which had fallen 
on the stigma were very few, while the ovules (in the same cap- 
sule) were most of them supplied with the organs described ; | 
concluded, therefore, that they were not at all connected with the 
pollen. The position of the exostome in reference to the apex of 
the placenta (it is directed to the base of that organ, and conse- 
quently away from the descending pollen-tubes) appeared also 
to present an insurmountable objection, unless we suppose the 
pollen-tubes to be possessed of a remarkable instinct, which 
I presume physiologists would hesitate to ascribe to them. 
They must, in order to reach the nucleus after descending, find 
their way to the exostome and then ascend, passing along the 
canal leading to the endostome. The smallness of the aperture 
in the outer membrane and its irregular margin would render 
it difficult for a pollen-tube to hit upon it exactly, keeping out 
of view altogether its direction. On the outside of the tubes 
described, were numerous spherical molecules, some of which 
were in active motion, and occasionally a few similar particles 
were seen in their interior. I afterwards found that these mo- 
lecules were abundant in the tissue of the placenta. The opi- 
nion first entertained about these organs (ovule-tubes), viz. that 
they had their origin from the pollen, was for these reasons 
abandoned. Supposing them to be prolongations of some part 
of the ovule itself, an objection having reference to the position 
of the exostome, and the passage of the tubes upward towards 
the apex of the placenta, might with great justice be urged; I 
found, however, that they in common have a great tendency to 
become tortuous and bent upon themselves : such a tube there- 
fore, on issuing from the exostome, and on being bent from its 
original direction, would come in contact with the placenta, and 
might have its further course regulated by contact with that or- 
gan. Without however laying much stress upon this, or ven- 
turing to speculate more in the matter, it may be remarked that 
the mode of growth of the tube would assist materially in regu- 
lating its course, especially if its increase in length took place at 
